Crystal growth is a very orderly and regulated process. Often many small crystals begin forming in solution and grow until their edges meet at varying angles. They do not join to form a single large crystal but rather remain a mass of small individual crystals forming a polycrystalline mass. Polycrystalline silicon metal (polysilicon) is a precursor to the manufacture of silicon single crystals used extensively in the microelectronics industry. Most of the current manufacturing schemes for the production of high purity polysilicon use metallurgical grade silicon as the starting material. This material is most often produced via a direct reduction of SiO2 in an electric submerged arc furnace. Coal and coke are used as carbon sources with sand or quartzite as the source of silica in the reaction:SiO2(s)+2C(s)⇄Si(s)+2CO(g)
The resulting metallurgical grade silicon (MGS) is further processed into gaseous precursors that are generally associated with polysilicon production. A typical process flow diagram for producing semiconductor grade polysilicon via a trichlorosilane (SiCl3H) intermediate was described by O'Mara, W, C., Herring, R. D., and Hunt, L. P., (1990) Handbook of Semiconductor Technology, Noyes Publications, Park Ridge, N.J., pp. 2-16, and is shown in FIG. 1.
In this process, ground MGS is treated with HCl to form trichlorosilane through a series of intermediate fractional distillation steps. The trichlorosilane can be directly reduced with H2 to produce polysilicon as in the Siemens process in which a mixture of trichlorosilane and hydrogen is guided, in a bell-jar reactor, over thin silicon rods which are heated by the passage of direct current to approximately 1110° C., producing polycrystalline silicon in the form of rods. Trichlorosilane can also be reacted with hydrogen in a series of reactions to form silane (SiH4). Silane is often preferred over trichlorosilane, despite the low silicon deposition rate for silane, due to the ease of purifying the silane gas in the production of high purity silicon.
There are many variations and intermediate products that are used in the industry to produce high purity polysilicon. Intermediate products such as SiCl2H2 are sold to manufacturers of polysilicon and wafer producers. However, most of the processes require a starting material of MGS, thereby requiring at least two separate operations; one to make the MGS and another to convert the MGS into polysilicon. In some instances three operations are required because the intermediate process to make trichlorosilane or silane is separated as a stand-alone operation. This is expensive and often requires the separation of the different processing steps into different facilities or locations. Thus, a less expensive process for the production of polysilicon metal which would allow the operation to be conducted in one facility is desired.